The present invention relates to a control scheme for power modulation of a free-piston Stirling engine-linear alternator power generator system. In particular, the present invention relates to a control scheme for maintaining high engine cycle efficiency over a wide operating range of a free piston Stirling engine-linear alternator generator system feeding power to a utility grid.
Although not limited thereto the present invention will be described in detail in connection with a control subsystem for a solar power conversion system arrangement in which a receiver is adapted to receive concentrated solar flux from a solar collector and convert it to thermal energy. The system includes a power module which converts the thermal energy supplied to the engine heater tubes efficiently into electrical power and feeds the electrical power to a utility grid.
The problem associated with solar power conversion systems is that as the solar insolation changes from the design level, the engine operation needs to be adjusted to avoid large changes in the engine heater temperature.
Power modulation can be effected by various approaches. The output power of a free piston Stirling engine is a function of the engine heat exchanger temperature ratio (heater/cooler absolute temperature ratio), operating frequency mean pressure, and volumetric displacement of the displacer and the power piston. However, power modulation by varying the heat exchanger temperature ratio is slow due to the high thermal inertia of the heat transport sub-system and the engine proper.
A change in mean pressure would require the transfer of working fluid between the engine and a storage bottle through solenoid valves. Engine power modulation by mean pressure control is not a preferred approach in terms of reliability and cost considerations.
Further, the operating frequency cannot be changed to change the power output since the free piston engine is connected to a utility grid which operates at grid voltage.
Typical schemes for varying volumetric displacement of the displacer and the power pistons include changing the relative phase between the displacer or power piston by varying the displacer or power piston gas spring stiffness. This is accomplished by varying the respective gas spring volumes. The displacer stroke can be changed by varying the displacer gas spring damping coefficient (by introducing a controlled leak through a valve between the gas spring volume and the engine mean volume). These power modulation approaches either change the relative phase or the stroke ratio between the power piston and the displacer. Since for a given engine geometry, the thermodynamic cycle efficiency is a strong function of the displace/power piston stroke ratio and relative phase angle, power modulation by the above schemes results in a significant reduction in cycle efficiency at off-design operating points.
Accordingly it would be advantageous to develop a new approach to power modulation which avoids the drawbacks associated with the aforementioned prior art proposals.